This invention relates to varactors, and more particularly, to metal-oxide-semiconductor varactors fabricated on complementary metal-oxide-semiconductor (CMOS) integrated circuits.
A varactor is a controllable capacitor. Metal-oxide-semiconductor (MOS) varactors are widely used in high frequency integrated circuits. For example, MOS varactors are commonly used as voltage-controlled tuning components in voltage-controlled oscillators. Voltage-controlled oscillators are used in circuits such as phase-locked loops. Phase-locked loops, in turn, are often used in circuits such as clock and data recovery circuits.
Successful operation of these circuits depends on accurate and predictable varactor performance. High performance varactors are characterized by high quality factors, a wide voltage-controlled capacitance range, and good linearity.
Previous attempts to improve on conventional MOS varactor designs have used so-called junction varactors, which do not have a wide voltage-controlled capacitance range. Other proposals have required significant changes to existing semiconductor processing techniques. Changes such as these are generally not commercially feasible.
It is therefore an object of the present invention to provide improved MOS varactors.